Some 50 activists, most from the capital’s eviction-hit communities, gathered in the city’s Boeng Kak neighborhood at about 8:30 a.m. to demand the release of four human rights workers and an election official who were jailed in May for allegedly bribing the mistress of deputy opposition leader Kem Sokha to deny the affair.

About 30 minutes later, dozens of Daun Penh district security guards—who gained notoriety for their violent repression of opposition protests in the wake of the 2013 election—showed up and destroyed a prop cage meant to symbolize the plight of the incarcerated five, said activist Song Sreyleap.

The situation escalated when the guards attempted to confiscate a drum used by the black-clad protesters.

“They destroyed our materials and pushed us to the ground,” Ms. Sreyleap said.

“They beat us, kicked us when we were on the ground, and punched a male protester twice in the back,” she said, adding that at least five activists sustained minor injuries, ranging from cut faces to sprained wrists.

An hour later in Dangkao district, about a dozen other protesters—who traded their usual black shirts for white in an attempt to convince authorities that they were not inciting a “color revolution,” as previously accused—attempted to travel to Prime Minister Hun Sen’s mansion but were also met with force.

When they boarded a convoy of tuk-tuks bound for the premier’s residence, a group of security guards ordered them out of the vehicles. Fifteen minutes later, police arrived on motorbikes and ripped apart their banners and balloons, shoving away activists who intervened.

“You are like hellish animals…you will get everything you have done to us!” she shouted through tears.

Contacted by telephone, Interior Ministry spokesman Khieu Sopheak threatened the “Black Monday” participants with legal action and claimed they were being funded by third parties, declining to elaborate.

“If they still refuse to follow orders [to stop protesting] one more time, we will take other action,” General Sopheak said. “We may send them to court.”

“To print T-shirts, they need money first. The ones who join protests are not the people who fund them. I know who gave them their money,” he said, going on to accuse the protesters of convincing their own children to cry for the cameras.

“They are professional demonstrators,” he said “They make the children cry like it’s the truth.”